Just filtered and treated tomorrow’s water, trying it again. I feel kind of left out, because I have never experienced the magic, purple water. 
If you give me your address I can send you a bottle full of purple water, or a bottle of water that will turn purple when you add Brewtan B to it. My starting water is municipal water that comes from the Hetch Hetchy system (Hetch Hetchy System ). This comes from snowmelt in the Sierra Nevada mountains, which is delivered to a local reservoir and is treated with chloramine. It is very low in mineral content but tends to be very high pH. I generally measure 9.3 - 9.6 pH, although sometimes I measure as low as 8.8. Because the mineral content is low it doesn’t take much in the way of dark grain or acid additions to lower the mash pH to the 5.2 - 5.6 range. I am not sure what causes the purple color, but I have seen it every time i have added Brewtan B and never otherwise.
The cause has been speculated about a few times on the forum. I think the front running hypothesis so far was some reaction with iron. You’ve neatly ruled that out it seems. pH might have something to do with it. Unlike most municipal supplies, mine is nearly neutral (~7.3.) The good thing is, nobody’s getting purple beer. It is a curiosity.
I have been using it in the mash and boil since it became available. I like the results. I feel I have increased break and the beers that I have bottled stay fresh longer. I do feel the clarity has improved too. I always felt that 6 months was the threshold for my lighter beers. I just finished my last sixer of Kolsch. I saved a case to try as it aged. I brewed it late Jan of 18. I thought it was still really good. Not like fresh no doubt, still totally drinkable. My bottling procedures have changed some, but at that age I have to think it is its use. I will continue to use it. I don’t see any reason I will be using it at packaging.
I used it both mash and boil again this weekend, first time in a long time using both doses (I’d used it in the mash again recently,) and I paid close attention. I can’t of course say for sure what it did in the mash, but it didn’t hurt, as far as I can tell. In the boil, it may well have enhanced the function of the Whirlfloc. Can’t be certain, but maybe. So many other factors. At any rate, wort from the lauter tun and into the fermenter were both crystal clear. I can get that otherwise, but. How much BTB contributed I don’t know, it doesn’t seem to have hurt. I suppose the takeaway is that BTB is at most one part of a whole regime, not a magic ingredient that will by itself radically improve your process or your beer. But as long as I have some around I suppose I’ll start tossing it in again. Thing is, I’ll probably never know if it extends shelf life. My beer is fermented in a closed process, kegged, stays cold its whole life, and that life is short. But it’s like yeast nutrient maybe. Cheap insurance you may not need.